It has been known that steelmaking slag produced in steelmaking process (e.g., converter slag, pretreatment slag, secondary refining slag and electric furnace slag) contains oxides of phosphorus, calcium, iron, silicon, manganese, aluminum, and magnesium, for example. Specifically, steelmaking slag contains phosphorus with calcium silicates such as Ca2SiO4 and Ca3SiO5, and calcium iron oxides such as Ca2Fe2O5. Steelmaking slag contains calcium, from quicklime (CaO) loaded during steelmaking process, as undissolved CaO (free lime), or as Ca(OH)2 or CaCO3 generated from free lime reacting with moisture or carbon dioxide in the air.
Phosphorus is an important element as the material for fertilizers or chemical products. Mineral phosphorus (phosphorus) is not produced in Japan, and imported in the form of mineral phosphorus, fertilizers, chemical products, for example. High-quality mineral phosphorus is low in quantity, which may cause strain on phosphorus resources; therefore, the phosphorus price is now on the rise (see, e.g., NPLs 1 and 2). In view of such a situation, when phosphorus can be recovered from the steelmaking slag, the strain on phosphorus resources would be alleviated. Thus, attempts to recover phosphorus from steelmaking slag have been made (see, e.g., PTLS 1 and 2).
PTL 1 discloses a method of recovering phosphorus from steelmaking slag with calcium removed. In the recovery method, calcium is removed from steelmaking slag by washing the steelmaking slag with water containing carbon dioxide. Next, phosphorus in the steelmaking slag is eluted into a mineral acid by dipping the steelmaking slag in the mineral acid. Lastly, phosphorus (phosphoric acid) is recovered by neutralizing the mineral acid containing eluted phosphorus (extract).
PTL 2 discloses a method in which a calcium compound from steelmaking slag for more than one times, and phosphorus in a state of solid solution in a specific calcium compound is recovered. In the recovery method, the steelmaking slag (pretreatment slag) is dipped in water containing dissolved carbon dioxide. Subsequently, a calcium compound having no phosphorus in a state of solid solution is eluted, and then, a calcium compound having phosphorus in a state of solid solution is eluted, whereby solution containing phosphorus is recovered from dephosphorization slag.
Calcium is also an important element which is used in a sintering process for iron-making in the form of calcium carbonate, or used in a steelmaking process in the form of calcium oxide after calcined. Calcium hydroxide obtained by slaking calcium oxide with water is used as a neutralizer for acids and the like in a draining process. Therefore, when calcium can be recovered from steelmaking slag obtained in iron-making process, calcium can be reused to reduce iron-making costs. Thus, attempts to recover calcium from steelmaking slag have been made (see PTL 3).
PTL 3 discloses a method of recovering calcium from converter slag using carbon dioxide. The recovery method elutes calcium from the converter slag by injecting water into the converter slag. Then, calcium (calcium carbonate) is recovered from the converter slag by keeping the lower limit of pH at about 10.